This invention relates to a safety device for an elevator, and more particularly to an inverter type safety device for an elevator.
An ordinary elevator is so designed that, when its cage reaches an aimed floor, its door is opened before the cage is completely stopped, and that if, when the cage is stopped at the floor, it is greatly vertically shifted from the floor because the cable is extended or contracted by the movement of people going into or coming out of the cage, then the cage is moved at extremely slow speed to reduce the vertical distance between the floor and the bottom of the cage. When this operation is carried out, the door is open. Therefore, if the control device becomes out of order during the operation, people may be dropped into the gap between the floor and the cage or at worst into the elevator shaft. Thus, when the control device becomes out of order, it is necessary to apply the emergency brake to stop the cage quickly to prevent the occurrence of such danger.
FIG. 3 is a diagram showing an operating characteristic in one example of a conventional elevator safety device disclosed, for instance, by Unexamined Japanese Patent Publication No. 115044/1977. In FIG. 3, reference character A designates the point where opening the door is started, and the curve EGO represents the speed of the cage when the latter reaches its aimed floor normally. The cage stops at the point O. The cage occupies a region indicated by lengths AO and OB (hereinafter referred to as "a cage landing region", when applicable) when it reaches the floor. Further in FIG. 3, a length AC or BD indicates the upper limit value of the cage speed when the cage reaches the floor. That is, the elevator safety device is designed as follows: In the case where after the cage has passed through the point A, a trouble occurs, so that the cage speed is going to exceed the upper limit at the point F as shown by the curve EGF, upon detection of the fact that the cage speed has exceed the upper limit, an emergency stop instruction is issued to stop the cage immediately.
The conventional elevator safety device is designed as described above; that is, after the detection of the fact that the cage speed has exceeded the upper limit, the emergency stop instruction is produced. Hence, the elevator safety device suffers from a difficulty that, for the period of time of from the occurrence of the trouble until the production of the emergency stop instruction the cage speed increases and the braking distance is increased accordingly. In the case of an elevator such as a gearless elevator having a large brake device, the period of time which lapses from the time instant the emergency stop instruction is issued until the brake torque is produced, namely, a braking period of time is generally long. Therefore, there has been a strong demand to detect the occurrence of a trouble quickly for the elevator.